In the prior art, scanning systems for electrophotographic copiers usually include a full-rate carriage and a half-rate carriage. The full-rate carriage, which supports a lamp and a mirror, moves along such a path as to illuminate successive portions of the document. The half-rate carriage supports one or more mirrors. It moves in the same direction as the full-rate carriage, but at one-half its speed. Light from the document is reflected by the full-rate mirror to the half-rate mirror and thence to a lens. The lens focuses the light upon a revolving photoconductive drum, producing a latent image from which a photocopy is made. The motion of the half-rate carriage relative to the full-rate carriage maintains a constant object distance from the illuminated portion of the document to the lens so that the image on the photoconductive drum is always in focus.
Scanning systems of the type described above must meet several requirements. During the scanning phase of the cycle, in which an image of the illuminated portion of the document is being projected onto the photoconductor, the motion of the scanning elements must be as nearly uniform as possible. Any jitter of the scanning elements during this phase of operation results in an undesirable light modulation, or banding, of the resulting copy, especially when copying low-contrast originals. A scanning system must also accelerate the scanning elements rapidly to a proper velocity during the start-up phase of the scanning cycle, as well as decelerate the scanning elements at the end of the forward scanning stroke. Likewise, the scanning should have a relatively short retrace time to achieve a high copy rate. Accelerations should not be so great, however, as to cause jitter or to damage the exposure lamp. Finally, the various scanning elements must be moved in close synchronism with one another as well as with the photoconductor.
To some extent, the various requirements discussed hereinabove conflict. Thus, generally in the prior art, gears, timing belts, chains, sprockets, and the like have been used to couple moving scanning elements to a drive mechanism. While such positive coupling elements allow relatively high-speed operation and insure synchronism between various elements so coupled, they also produce a cogging effect, impairing the smoothness of movement that is required during the scanning phase of the cycle. Furthermore, if massive scanning elements are used to smooth out momentary speed fluctuations, such elements limit the maximum acceleration or deceleration, undesirably reducing the copying rate.
It is known in the art to couple scanning carriages to the photoconductor drum during the forward scanning stroke for synchronous movement with the drum and to uncouple the carriages from the drum at the end of the forward stroke to permit a spring to return the scanning carriages to their original positions. Although such a scanning system has a lower inertia during the return stroke than during the forward scanning stroke, it does not completely solve the problems referred to above. In such a system, the mass of the continuously rotating photoconductor drum is used to accelerate the scanning elements almost instantaneously from the rest position at the beginning of each scanning cycle. Such an instantaneous acceleration gives rise to large reaction forces and vibrations in the drive train, which impair the smoothness of scanning motion. Further, such an arrangement is unsuitable, without modification, for use in a variable-magnification copier, since the scanning elements must be moved at variable speeds, possibly over a continuous range, relative to the speed of the photoconductor.
Cail et al U.S. Pat. No. 4,332,461 discloses a scanner drive for a variable-magnification copier in which a separate scanner motor is smoothly accelerated at the beginning of the scanning cycle to the desired scanning speed at a controlled rate of acceleration, and is phase-locked to the photoconductor motor, at a velocity ratio determined by the selected magnification, during the constant-velocity portion of the scanning cycle. Although the disclosed system is said to be capable of substantially continuously variable magnification and to eliminate direct mechanical couplings between the photoconductor drive and the scanner drive, it does not completely solve the problems of the prior art. In particular, the servo system disclosed, while locking the scanning velocity to the photoconductor velocity, cannot be relied upon to eliminate momentary fluctuations in scanning velocity. Moving scanning elements massive enough to provide the required inertial smoothing would also undesirably reduce the scanning rate achievable with a given motor torque.
Scanning carriages are usually mounted on two parallel guides and are driven by a single cable, attached to the full-rate carriage and engaging a pulley mounted on the half-rate carriage. The cable and the pulley are mounted on one side of the carriages, adjacent one of the guides, to avoid obstructing the light path from document to lens. Since the cable and pulley do not act through the centers of gravity of the carriages, appreciable moments are applied to the carriages, tending to cock them. To resist cocking, each carriage is provided with guide bearings having a large spacing along at least one of the guides, increasing the length and weight of the carriages. The carriages must have a rigid and hence massive structure to resist distortion under the asymmetrical driving forces. Acceleration of the carriages at the beginning and end of a scan produces large frictional forces in the guide bearings which tend to cause chattering.
It is also known in the art to use separate, transversely spaced cable drives, coupled to the scanner carriages along their respective sides. Although such an arrangement eliminates the rotational reaction forces developed by the scanner carriages upon acceleration, it introduces the possibility that a misalignment of the drive cables will produce a corresponding shearing of the scanner carriages.